President Barack Obama speaks during an interfaith healing service Thursday at the Cathedral of the Holy Cross in Boston for victims of Monday's Boston Marathon explosions. (ASSOCIATED PRESS / Charles Krupa)

BOSTON (AP) — Called upon to console a grieving city and reassure a shaken nation, President Barack Obama on Thursday promised that Boston would “run again” after deadly twin bombings at its famous marathon. More than 2,000 people rose in a standing ovation in the Cathedral of the Holy Cross and sang “America the Beautiful.”

Obama’s message of resolve in time of tragedy was echoed by Mayor Thomas Menino and Gov. Deval Patrick at a packed interfaith service.

“Nothing will take us down because we take care of one another,” Menino said. “Even with the smell of smoke in the air and blood in the streets and tears in our eyes, we triumphed over that hateful act.”

Three people were killed and more than 170 others were injured, some of them grievously, in Monday’s bombings near the race’s finish line.

Obama spoke just hours before the FBI released photographs and video of two suspects and asked for the public’s help in identifying them.

In the midst of an emotional and trying stretch for the country and his presidency, Obama vowed to track down those responsible and lauded Boston’s “undaunted” spirit.

“Your resolve is the greatest rebuke to whoever committed this heinous act,” he told the gathering.

Indeed, Obama’s words underscored the stark reality that has left many Americans jittery. Even as authorities narrowed their search to two men captured on video, the suspects remained at large and their motives were still a mystery.

For Obama, the bombings began a week consumed not only by terror but also disaster and political defeat. Letters sent to Washington officials, including Obama and Sen. Roger Wicker, R-Miss., were found to contain traces of poisonous ricin in tests, evoking eerie parallels to the anthrax attacks that followed the terror of Sept. 11, 2001.

The president also lost a fight for new gun control measures in the Senate, then awoke Thursday to news of a powerful fertilizer plant explosion that devastated a small Texas town.

Speaking from the pulpit in the soaring cathedral, the president didn’t explicitly declare the deadly marathon explosions an “act of terror” as he did earlier in the week during remarks at the White House. But he showed little restraint in describing those responsible for the attack, calling them “small, stunted individuals.”

“Yes, we will find you, and yes, you will face justice,” he said, as the crowd — some wearing bright yellow marathon jackets — applauded.

Also in the crowd was Obama’s former presidential rival, Mitt Romney, who served one term as Massachusetts governor. Several state officials, including Democratic Sens. Elizabeth Warren and William “Mo” Cowan, traveled to Boston with Obama on Air Force One, as did Vicki Kennedy, the widow of the late Massachusetts icon, Sen. Ted Kennedy.

The president spoke of Boston in personal terms, reminding the audience of the years he spent in the city as a student at Harvard Law School. Boston was also the host for the 2004 Democratic National Convention that featured Obama as the keynote speaker, a role that would thrust the little-known Illinois state senator into the national spotlight.

Obama and first lady Michelle Obama sat at the front of the church next to Patrick. A mournful string solo by cellist Yo-Yo Ma preceded the governor’s own remarks.

“We will grieve our losses and heal,” Patrick said. “We will rise, and we will endure. We will have accountability without vengeance, vigilance without fear.”

Thursday’s service included reflections by representatives of Protestant denominations, the Jewish, Muslim and Greek Orthodox faiths, and Cardinal Sean O’Malley, head of the Roman Catholic church in Boston.

The event was open to the public on a first-come, first-seated basis, but the line to try to get a ticket stretched at least two city blocks. There was a heavy police presence around the cathedral in the city’s South End, and authorities closed nearby streets to traffic.

“I think it’s important that we heal as well as those who were affected. I guess sometimes you feel you can’t do a single thing, and this is something we can do,” said Beth Anne Stevenson, a Boston Medical Center surgical intensive care unit nurse.

Ricky Hall, 67, of Cambridge, showed up at 8 a.m. and stayed even after he believed he wouldn’t get inside.

“I came to pay my respects to the victims,” he said.

Following the 90-minute service, Warren urged Bostonians not to succumb to fear.

“We will be vigilant but we will not be afraid,” she said.

The Obamas met privately with the family of Krystle Campbell, 29, who was among those killed in the attack. The Obamas also visited with many of the injured in their hospital rooms at Massachusetts General Hospital and thanked the marathon volunteers who rushed toward danger to help the wounded.

Separately, the first lady met with patients, their families and hospital staff at Boston Children’s Hospital and Brigham and Women’s Hospital.

Obama has played this role before, serving as the nation’s consoler in chief at times of heartbreaking tragedy. He’s grieved with the families of those killed in shootings at Newtown, Conn., Tucson, Ariz., Aurora, Colo., and Fort Hood, Texas. And he’s comforted residents in tornado-ravaged Missouri and storm-ravaged communities in New York and New Jersey.